1. Field of the Invention
This invention relates to an optical amplifier used, for example, in an optical fiber communication system. Specifically, this invention relates to a reflection type of optical fiber amplifier using an optical circulator.
2. Description of the Related Art
Related Art 1
FIG. 9 shows a configuration of a conventional optical amplifier as disclosed in Japanese Unexamined Patent Publication No. HEI 6-324368.
In FIG. 9, an optical circulator 1, which is an optical transmission device, has input/output terminals 1a to 1d. The input/output terminal 1a connects an optical fiber 5a, which is a signal light input. The input/output terminal 1b connects an optical fiber 51. The input/output terminal 1c connects an optical fiber 52. The input/output terminal 1d connects an optical fiber 5b, which is a signal light output. The optical fiber 51 connects an erbium-doped optical fiber (referred to as EDF, hereinafter) 2a, which is the first light amplification fiber, and an optical filter 3a made of dielectric multilayer film, which is the first optical filter and a semiconductor laser module (referred to as LD/M, hereinafter) 4a for generating 0.98 .mu.m pump light, which is the first pump light source. The optical fiber 52 connects an EDF 2b, which is the second light amplification fiber, and an optical filter 3b made of dielectric multilayer film, which is the second optical filter, and an LD/M 4b, which is the second pump light source.
Next, the operation of the optical amplifier of FIG. 9 will be described. A pump light, whose wavelength is 0.98 .mu.m, is output from the LD/M 4a and input to the EDF 2a through the optical fiber 51 and the optical filter 3a. Then, the pump light of 0.98 .mu.m leads erbium ions in the EDF 2a to an excited state and light amplification operation for the light of 1.5 .mu.m band is performed in the EDF 2a. Likewise, a pump light, whose wavelength is 0.98 .mu.m, is output from the LD/M 4b and input to the EDF 2b through the optical fiber 52 and the optical filter 3b. Then, the pump light of 0.98 .mu.m leads erbium ions in the EDF 2b to an excitation state and light amplification operation for the light of 1.5 .mu.m band is performed in the EDF 2b.
For example, a signal light, whose wavelength is 1.55 .mu.m, is input to the optical circulator 1 through the optical fiber 5a and the terminal 1a. Then, a signal light, whose wavelength is 1.55 .mu.m, is output from the terminal 1b and passes the EDF 2a through the optical fiber 51. The signal light is amplified by passing the EDF 2a. After that, the signal light is reflected by the optical filter 3a and amplified again by reversing the EDF 2a. Consequently, the signal light is again input to the circulator 1 from the terminal 1b. Then, the amplified signal light is output from the terminal 1c and amplified by going through the EDF 2b, the optical filter 3b, the EDF 2b, and the terminal 1c orderly as stated before. The amplified signal is output from the terminal 1b to the optical fiber 5b. The optical circulator transmits light between terminals in a specific direction such as from 1a to 1b, from 1b to 1c, from 1c to 1d and from 1d to 1a. However, between the terminals in the other directions, the light is not substantially transmitted. In addition, there is an optical circulator having three terminals and a configuration where only one EDF is used is known.
Related Art 2
According to a "Discussion of optical fiber amplifier for WDM electrical transmission system using a B-1099 hybrid EDF" by Tomonori Kashiwada and the other five people in the general meeting of the Institute of Electronics Information and Communication Engineers in 1995, it is reported that a hybrid EDF which is a cascade connection of P-Al(P stands for phosphorus, Al stands for aluminum)-codoped EDF and Al(Al stands for aluminum)-doped EDF can make gain flat, in a wide wavelength range whose signal wavelength is from 1543 to 1558 nm without degrading amplification efficiency, comparing to the Al-doped EDF simple alone.
FIG. 10 shows an output spectrum of the hybrid EDF in cascade connection of P-Al-codoped EDF and Al-doped EDF. The Hybrid EDF of FIG. 10 inputs a signal light from the side of P-Al-codoped EDF and inputs a pump light from the side of Al-doped EDF. For the spectrum in FIG. 10 showing gain as a function of wavelength, the total input signal light is -15 dBm and the excitation wavelength is 1.47 .mu.m. FIG. 10 shows a slope .theta. of gain obtained from the output spectrum whose signal light wavelength is 1543, 1548, 1552, and 1558 nm. Each gain is almost the same and the slope .theta. is close to zero.
FIG. 11 shows an output spectrum at a simultaneous amplification of four waves (1543/1548/1552/1558 nm) by Al-doped EDF under the same conditions for FIG. 10. In FIG. 11, a slope .alpha. of gain from the output spectral whose signal light wavelength is 1543, 1548, 1552 and 1558 nm is shown.
FIG. 12 shows an output spectrum at a simultaneous amplification of four waves (1543/1548/1552/1558 nm) by P-Al-codoped EDF under the same conditions of FIG. 10. In FIG. 12, a slope .beta. of gain from the output spectral whose signal light wavelength is 1543, 1548, 1552 and 1558 nm is shown.
In FIG. 11, the gain of the signal light on the longer wavelength is higher and a maximum of difference of gain between the four waves is about 3 dB. In FIG. 12, by using P-Al-codoped EDF, a contrary amplification characteristic to Al-codoped EDF can be obtained when a wavelength is from 1.54 to 1.56 .mu.m.
When cascade connection of P-Al-codoped and Al-doped EDF is configured, the slope .alpha. of gain for the wavelength by Al-doped EDF and the slope .beta. of gain for the wavelength by P-Al-codoped EDF are offset each other. Therefore, as shown in FIG. 10, the slope becomes .theta. and wavelength dependency of gain disappears. FIG. 13 shows wavelength dependency of output at the time of a simultaneous amplification of four waves. In FIGS. 10 and 13, the total output power of the hybrid EDF is +14.2 dBm, which is almost equal to that of Al-doped EDF of 22 m in length. However, a maximum difference of gain between four waves by hybrid EDF is 1.3 dB and this value (1.3 dB) almost corresponds to that of Al-doped EDF of 13 m in length.
Related Art 3
According to "B-1067 optical preamplifier having auto gain control function" by Katsumi Takano and the other four people in the general meeting of the Institute of Electronics Information and Communication Engineers in 1995, a cascade connected 0.98/1.48 .mu.m coexcitation type of optical fiber amplifier is explained. FIG. 14 shows a configuration of an optical preamplifier. In FIG. 14, the former EDF 900 performs 0.98 .mu.m (980 nm) forward excitation in order to reduce noise. The latter EDF 901 performs 1.48 .mu.m (1480 nm) backward excitation. The former EDF 900 is 12 m in length and the back step EDF 901 is 40 m in length. An Automatic Gain Control (AGC) circuit 902 controls outputs of a 980 nm Laser Diode (LD) and a 1480 nm LD in order to make data amplitude of a receiving circuit stable.
Related Art 4
On page 180 of "Optical amplifier and its application" published by Ohmu-sha, a laser module using the non-spherical lens is shown. The laser module using the non-spherical lens as shown in FIG. 15 has a case 910 fixing the non-spherical lens 914 inside and a package putting a laser 915 inside. The case and the package are fixed by welding and integrated.
In an optical amplifier using an optical circulator as shown in Related Art 1, one optical circulator has input/output terminals for the optical amplifier and prevents reversing of lights and has a function for providing a stable amplification operation. Therefore, a simplified configuration can be enabled by using a fewer number of elements. According to an excitation by 0.98 .mu.m pump light, as known, three-level of excitation is enabled, and a low noise characteristic can be realized. Further, energy conversion efficiency of a pump light and a signal light is calculated according to ratio of each wavelength of the light. In Japanese Unexamined Patent Publication No. HEI 6-324368, as above stated, the wavelength of the signal light is 1.55 .mu.m. Therefore, energy conversion efficiency is 0.98/1.55=0.63. That is, there is a problem that the energy conversion efficiency cannot be more than 63% in principle.
The EDF 2a and the EDF 2b as shown in Related Art 1 provides light amplification operation of wavelength range, from about 1.52 to 1.58 .mu.m. However, amplification rate, that is, a gain depends on the wavelength of the signal light. FIG. 16 shows ASE (Amplified Spontaneous Emission) spectral in erbium-diffused optical fiber where erbium is diffused in pure silica (SiO.sub.2) core (FIG. 16 shows a figure on page 115 of "Optical amplifier and its application" published by Ohmu-sya). FIG. 16 shows a light strength (hereinafter, referred to as power) to a signal light whose wavelength is from 1.515 .mu.m to 1.565 .mu.m according to a 0.98 .mu.m pump light. In FIG. 16, the power has peaks at the wavelength of 1.536 .mu.m and 1.552 .mu.m. On the other hand, the power is at least at wavelength of 1.515 .mu.m in the figure. Then, the difference of the power between the wavelength of 1.536 .mu.m and that of 1.515 .mu.m is almost 30 dB.
Therefore, when a plurality of signal lights of different wavelengths are input, an unbalance in the power of each output signal light results.
As a hybrid EDF, a hybrid EDF which has cascade connection of P-Al-codoped EDF and Al-doped EDF was described. However, there is a problem that adjustment of length of EDF is required so that a slope of gain by P-Al-codoped EDF and a slope of gain by Al-doped EDF can be offset.
In a cascade-connected 0.98/1.48 .mu.m coexcitation type of optical fiber amplifier in Related Art 3, high output is realized according to 1.48 .mu.m backward excitation. However, since excitation power is limited, fiber length for gaining a maximum gain should be lengthened. When the fiber length is lengthened, there causes a problem that an optical fiber amplifier should be large in scale.